Organic EL elements are semiconductor elements that convert electrical energy to optical energy. In recent years, research using organic EL elements have been actively made, and the practical use thereof has been advancing. The organic materials constituting organic EL elements, and other aspects of organic EL elements have been improved, thereby lowering the driving voltage of the elements remarkably and further enhancing the light-emitting efficiency thereof. In the market, televisions wherein an organic EL element is used as a display screen have also been sold.
In order to make the luminance of an EL element higher, a high electric field is applied to the element to make a current density high. However, by making the current density high, the quantity of generated heat increases, causing a problem that deterioration of the organic thin film itself is promoted. In order to solve this problem, it is necessary to raise the luminance of light emitted without changing the driving current.
In contrast, it has been recently reported (see, for example, Patent Documents 1 to 3) that plural light-emitting layers for an organic element are stacked on each other, and the layers are connected to each other in series, thereby making it possible to make the luminance of the element high. Patent Document 4 discloses a stacked-type organic light-emitting element wherein an electrically insulating charge-generating layer containing a metal oxide such as vanadium pentaoxide (V2O5) is arranged between plural organic light-emitting units. Patent Document 5 suggests the use of a change-generating layer in which molybdenum trioxide is used instead of vanadium pentaoxide.
When an electric field is applied to such an organic light-emitting element wherein a charge-generating layer is arranged between light-emitting units, the charge-generating layer simultaneously generates holes that can be injected into a hole-transporting layer arranged on the cathode side, and electrons that can be injected into an electron-transporting layer arranged on the anode side. For this reason, the plural light-emitting units act like light-emitting units connected to each other in series through the charge-generating layers. Such a stacking method is called multi-photon emission (MPE).
For example, Patent Documents 3 and 4 disclose that a radical anion-containing layer made of Alq:Liq/Al is used as an anode-side layer of such a charge-generating layer. In this structure, Li ions in Liq are reduced by a thermally reducing metal such as Al, and the resultant acts as a radical anion-generating means; thus, an electron-transporting organic substance such as Alq is present in a radical anion state so that electrons which can be injected into the electron-transporting layer are generated.
In the structure where plural light-emitting units are connected to each other, a layer arranged between the light-emitting units is variously named. In the present specification, however, a region that is sandwiched between plural light-emitting units in order to connect the two light-emitting units to each other is called a “connection layer”. For the structure of this “connection layer”, various forms are suggested besides the structures disclosed in Patent Documents 3 and 4 (see, for example, Patent Documents 5 to 7). When the connection layer has a multi-layered structure, an extra voltage is generated in the connecting region in accordance with the structure of the individual layers or the stack order thereof. Thus, this voltage may cause inconveniences such as that the connection layer becomes unstable, and that reliability for long lifetime cannot be obtained.
For example, when vanadium oxide is arranged on the anode side of a charge-generating layer, stoichiometry of the material thereof is important. If its composition ratio departs therefrom, the charge-generating layer becomes unstable. The unstable charge-generating layer reduces the function thereof remarkably. In order to enable a connection layer to act stably as a charge-generating layer, it is known that an interfacial structure on the anode-side of the connection layer is very important.
In the meantime, Patent Document 7 discloses that a layer made of an oxide of an alkali metal or alkaline earth metal is arranged on the anode side of a connection layer. In this oxide layer, metal ions of the oxide act as an electron-donating dopant, so that this layer has an effect of improving the efficiency of injecting electrons to the light-emitting unit present on the anode side of the connection layer. However, when a metal oxide such as lithium carbonate (Li2CO3) is produced, metal ions easily diffuse to the organic layer. As a result, it is feared that the lifetime of the element is made short.